Fluorescent material and method of preparing same



Patented Oct. 12, 1926.

UNITED STATES PATENT OFFICE.

SAMUEL E. SHEPPARD. OF ROCHESTER, NEW YORK, ASSIGNOR TO EASTMAN KODAK COMPANY, OF ROCHESTER, NEW YORK,v A CORPORATION OF NEW YORK.

FLUORESCENT MATERIAL AN D METHOD OF PREPARING SAME.

' No Drawing. Continuation of application Serial No. 28,003, filed May 4, 1925.

February 17, 1926.

This invention relates to improvements in the material used as the active fluorescent agent in X-ray intensifying screens. Awell known and undesirable characteristic .of the salt principally used, calcium tungstate, is its tendenc to give phosphorescence on exposure to -rays as well as fluorescence. That is, it will continue to emit light after the exciting rays have ceased. The degree of this fault varies with different batches of material. Various methods of preparing very pure tungstate to overcome this trouble have been described, but they are tedious, costly and uncertain. I have found that this phosphorescence, which is generally called after-glow or lag, may be markedly reduced by incorporating a small amount of certain vanadium compounds in the calcium tungstate. Thequantities necessary to reduce phosphorescence to an innocuous point are very small and do not appreciably affect or reduce the fluorescence.

The vanadium compound may'be introduced during the manufacture of the tungstate by any of the usual or known methods or may be added in aqueous solution to the tungstate after manufacture by any customary process.

One of the known methods of making calcium tungstate is by the fusion of calcium chloride and sodium tungstate at a temperature of the order of between 1800 and 2000 F. In this process I add the vanadate, usually the sodium or ammonium salt of vanadic acid, before fusion and carry on the process in the usual way, quenching the fused mixture in watg and washing it well. A suitable quantity of the 'vanadate is y from .2 to .5 per cent based on the dry weight of the calcium-tungstate produced. Smaller quantities than this may be used, producing eifective repression of phosphorescence (equal to present standards) but not such complete elimination, while the addition of more serves no useful purpose. The optimum quantity will vary with different batches of material and must be controlled by test.

The addition of the vanadium compound, is however, equally effective when other methods of preparing the calcium tungstate are employed.

As stated above, the vanadium compound may be added to the tungstate after its pro- This application filed Serial No. 88,953.

duction by the method referred to above or by any other method. The proportion of the vanadate to the tungstate is the same as was mentioned above. An aqueous solution of the vanadate is added to the prepared tungstate, with which it is well mixed and the mixture then dried.

While for commercial and practical reasons, I use the salts mentioned, the vanadates of any of the alkali metals (including ammonium) and of the alkaline earthmetals are substantially equally effective. I may also use vanadic acid or any compound of vanadium that will yield vanadic acid on oxidation, such as the sulphate or halides.

The same vanadium compounds may be used in the similar treatment of and in the similar process of preparation of cadmium tungstate, or a mixture of calcium and cadmium tungstate, such as is used more especially for-fluoroscopic screen for visual examination as distinguished from the intensifying screens of calcium tungstate used with photographic plates. I

This application is a continuation of my aiipggication, Serial No. 28,003, filed May 4:,

Having thus described my invention, what I.claim as new and desire to secure by Letters Patent is I 1. A composition of matter in which has been incorporated a tungstate having high fluorescent properties when excited by X- rays and a compound of vanadium which is capable, underoxidizing conditions, of insuring the presence of vanadic acid.

2. composition of matter in which has been incorporated a tungstate having high fluorescent properties when excited by X- rays and a vanadate of an alkali metal or of an alkaline earth metal.

8. A composition of matter in which has been incorporated a tungstate having high fluorescent properties when excited by X- rays and a vanadate of an alkali metal.

4. A composition of matter in which has been incorporated two hundred parts of a tungstate having high fluorescent properties when excited by X-rays and less than one part of a vanadate of an alkali metal.

5'. A composition of matter for use in X- ray screens in which has been incorporated calcium tungstate and a vanadate of an alkali metal.

6. The method of reducing the hosphorescent properties of a highly, uorescent tungstate that comprises the incorporation therewith of a vanadium compound capable of insuring the presence of Vanadic acid under oxidizing conditions.

7. The method of reducing the phosphorescent properties of a highly fluorescent tungstate that comprises the incorporation therewith of a vanadate of an alkali metal or of an alkaline earth metal.

8. The method of reducing the phosphorescent properties of a highly fluorescent tungstate that comprises the incorporation therewith of a vanadate of an alkali metal.

9. The method of reducing the phosphorescent properties of calcium tungstate that comprises incorporating therewith the vanadate of an alkali metal.

Signed at Rochester, New York this 12th day of February 1926.

SAMUEL E. SHEPPARD. 

